1. Field of the Invention
This invention relates to a misfire-detecting system for internal combustion engines, and more particularly to a misfire-detecting system of this kind, which is equipped with a device for detecting abnormality thereof.
2. Prior Art
A spark plug of an internal combustion engine can suffer from abnormal ignition or misfires. The misfires are largely classified into ones attributable to the fuel supply system and ones attributable to the ignition system. Misfires attributable to the fuel supply system are caused by the supply of a lean mixture or a rich mixture to the engine, while misfires attributable to the ignition system are caused by so-called mis-sparking, which means that normal spark discharge does not take place at the spark plug. Mis-sparking occurs e.g. due to smoking or wetting of the spark plug with fuel, particularly adhesion of carbon in the fuel to the spark plug, which causes current leakage between the electrodes of the spark plug, or due to abnormality in the sparking voltage-supplying system.
The present assignee has already proposed a misfire-detecting system for detecting misfires attributable to the fuel supply system, which comprises sparking voltage-detecting means for detecting sparking voltage across electrodes of a spark plug, and misfire-determining means which determines that a misfire has occurred when a time period over which the detected value of the sparking voltage exceeds a predetermined voltage value exceeds a predetermined time period (Japanese Patent Application No. 3-326507).
However, the above proposed system is incapable of detecting abnormality in the system, such as a fault in a sensor for detecting the sparking voltage, or a disconnection or a ground-fault (short-circuit to ground) in wiring between the sensor and an electronic control unit for executing the misfire determination.